1. Field of the Invention
The present invention relates to optical fiber built-in type composite insulators which are mainly used in detection systems for finding fault points at electric power transmission lines and transformer substations, etc., and a method of producing the same.
2. Related Art Statement
Heretofore, in order to automatically find fault points in electric power systems, optical fiber built-in type composite insulators have been used which are capable of transmitting signals from optical sensors at a power supply side to detectors at a grounded side and maintaining an electrical insulative property.
Various types of optical fiber built-in type composite insulator are known. Among them, a technique is known wherein a penetration hole is bored in the axis of a ceramic insulator body, one or two optical fibers are inserted in the penetration hole, and a portion or the whole of the penetration hole is filled with an organic insulative material, such as silicone rubber or epoxy resin, etc., to seal the optical fiber in the penetration hole and prevent decrease of creepage distance of the insulator. Also, a technique is known wherein the whole ceramic insulator having a penetration hole and an optical fiber therein are heated and a molten glass is poured in the whole or a portion of the penetration hole to seal the optical fiber in the penetration hole.
However, in the abovementioned sealing with an organic material, the organic sealing material and the ceramic insulator body have a such a large difference in thermal expansion coefficients from each other, that problems occur in that deterioration of the organic sealing material is accelerated and the optical fiber is occasionally broken by a thermal stress generated by temperature change while in use. Moreover, the organic sealing material has a problem in that it is liable to incur tracking, etc., during a long use, so that it has a poor reliability when used for a long time.
Also, in the abovementioned sealing with the inorganic material or glass, the whole ceramic insulator has to be heated, so that problems occur in that a large apparatus is required to increase the investment cost and a large amount of electric power is consumed, and increases costs. Moreover, when heating the whole insulator and the optical fiber for melting the glass, problems occur in that the coating of the optical fiber is scorched, so that the optical fiber is liable to break down and the structure of extending the optical fiber from the ends of the insulator can hardly be obtained. For obviating these problems, surfaces of the optical fiber exposed at the end surfaces of the insulator after sealed by the glass have to be optically polished and adhered by means of Ferrule, etc., so that other problems such as complicated and expensive production steps arise.
In order to solve the above problems, the applicant proposed in their Japanese Patent Application Laid-open No. 1-246,724 (U.S. Pat. No. 4,921,322) two sealing methods, as shown in the attached FIGS. 4 and 5.
In the sealing method as shown in FIG. 4, an insulator 31 and an optical fiber 3 are fixed by jigs 26A, 26B for fixing the insulator 31 and jigs 24A, 24B for fixing the optical fiber 3. These jigs are constructed in such a fashion that their vertical and horizontal spacings can be adjusted depending on the positions of the insulator 31 and the optical fiber 3. For the insulator 31 which has finished preliminary heating thereof are arranged an induction heating furnace 21 for melting a glass, a hot air blower pipe 22, and a cooling pipe 23. Next, an upper end of the insulator 31 is heated by hot blow of a temperature of, for example, 550.degree. C.+20.degree. C., from the hot air blower 22 for 5 min., and then filled with a sealing glass of a desired composition melted at, e.g., 500.degree. C. in the induction heating furnace 21 to a sealing portion in the penetration hole. After filling a desired amount of the sealing glass to finish the sealing operation at the end of the insulator 1, the insulator 1 is turned over and the same glass-filling operation as described above is performed on the lower end portion of the insulator 1 to complete the sealing process. The cooling pipe 23 is used for preventing heating of the jigs 24A, 24B which fix the optical fiber 3.
However, even the method of FIG. 4 has the following drawbacks. Namely, when the inorganic glass is filled in the sealing portion after melted by heating, the neighboring portion of the ceramic insulator 1 around the inorganic glass can be heated below the temperature of the inorganic glass and insufficiently expanded, though the glass is preliminarily heated by the hot blow. As a result, when the inorganic glass is cooled and solidified, a tensile stress is exerted on the inorganic glass and the neighboring portion of the ceramic insulator 1, so that a crack is liable to form in the sealing inorganic glass. Moreover, it is difficult to keep the heated and melted inorganic glass in a constant state. Furthermore, when filling the heated and melted inorganic glass in the sealing portions of the penetration hole, there is a high risk of damaging the optical fiber 3, such as scorching the coating portion of the optical fiber 3.
In the sealing method as shown in FIG. 5, a preliminary sealing member 41 is formed at first. That is, at a position of the optical fiber 3 corresponding to the end portion of the penetration hole 2, an electrically conductive ceramic or metallic tube 37 having an outer diameter capable of being inserted in the penetration hole 2 is provided, and a spacer 5 and a sealing glass 34 are provided in the tube 37 to form the preliminary sealing member 41 for sealing the optical fiber 3 therein.
Next, the optical fiber 3 with the preliminary sealing member 41 therearound is inserted in the penetration hole 2 of the insulator 1 to locate or position the preliminary sealing member 41 at the end portion of the penetration hole 2, as shown in FIG. 5. At this time, a sealing glass 34 preferably of a paste state should be intervened between the outer circumferential surface of the tube 37 of the preliminary sealing member 41 and the inner circumferential surface of the penetration hole 2. Thereafter, a high frequency induction heating device 42 is positioned at a position corresponding to the end portion of the penetration hole 2 and high frequency induction heating is effected. The electrically conductive ceramic or metallic tube 37 is induction heated, so that the sealing glass 34 arranged between the outer circumferential surface of the tube 37 of the preliminary sealing member 41 and the inner circumferential surface of the penetration hole 2 is melted to complete the sealing operation. Thereafter, a protecting member for protecting the sealed end portion, such as silicone rubber, etc., is provided on the sealed end portion around the optical fiber 3.
However, even the method of FIG. 5 has drawbacks in that the portion of the ceramic insulator 1 around the inorganic glass can be heated below the temperature of the inorganic glass, so that a thermal stress is generated between the inorganic glass 34 and the neighboring portion of the ceramic insulator 1 to occasionally form a crack in the sealing inorganic glass 34 when the glass 34 is cooled and solidified. Moreover, the inorganic glass 34 used between the ceramic insulator 1 and the electrically conductive ceramic or metallic tube 37 is liable to peel from the tube 37 at the bonded interface thereof than from the insulator 1 at the bonded interface thereof during a long use, so that it has poor reliability of the bonding portion. Furthermore, positioning the tube 37 at a desired position in the penetration hole 2 is difficult and may damage optical fiber 3 and the tube 37, which are difficult to can hardly be uniformly heat. Furthermore, the tube 37 has to be formed to a desired shape beforehand, a high frequency induction heating device has to be used for heating the tube 37, and the inorganic glass 34 has to be applied, calcined and baked on the tube 37, so that production steps are difficult, cumbersome and too numerous.